Semiconductor device and method of manufacturing semiconductor device

ABSTRACT

A semiconductor device includes a semiconductor layer. A first conductivity type region is formed on a base layer portion of the semiconductor layer. A body region of a second conductivity type is formed on the semiconductor layer to be in contact with the first conductivity type region. A trench in which a gate electrode is embedded through a gate insulating film is formed on the semiconductor layer. The trench penetrates through the body region, so that a deepest portion thereof reaches the first conductivity type region. A source region of the first conductivity type is formed on a surface layer portion of the semiconductor layer around the trench. The gate insulating film includes a thick-film portion having a relatively large thickness on a bottom surface of the trench.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 15/969,770,filed on May 2, 2018, and allowed on Oct. 4, 2018, which is acontinuation of application Ser. No. 15/223,037, filed on Jul. 29, 2016(now U.S. Pat. No. 9,978,860, issued on May 22, 2018), which is acontinuation of application Ser. No. 13/371,501, filed on Feb. 13, 2012(now U.S. Pat. No. 9,406,794, issued on Aug. 2, 2016), which is adivisional of application Ser. No. 12/230,620, filed on Sep. 2, 2008(now U.S. Pat. No. 8,129,779, issued on Mar. 6, 2012). Furthermore, thisapplication claims the benefit of priority of Japanese applications2007-228033 filed on Sep. 3, 2007, 2007-244410 filed on Sep. 20, 2007,and 2008-220163 filed on Aug. 28, 2008. The disclosures of these priorU.S. and Japanese applications are incorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a semiconductor device having a trenchgate type VDMOSFET and a method of manufacturing the same.

Description of Related Art

A trench gate structure is generally known as a structure effective forrefinement of a VDMOSFET (Vertical Double diffused Metal OxideSemiconductor Field-Effect Transistor).

FIG. 7 is a schematic sectional view showing the structure of aconventional semiconductor device having a trench gate type VDMOSFET.

The semiconductor device 100 includes an N⁺-type substrate 101. AnN⁻-type epitaxial layer 102 is laminated on the N⁺-type substrate 101.Abase layer portion of the N⁻-type epitaxial layer 102 forms an N -typeregion 103, while a P-type body region 104 is formed on a surface layerportion of the N⁻-type epitaxial layer 102 vertically adjacently to theN⁻-type region 103.

A trench 105 is dug in the N⁻-type epitaxial layer 102 from a surfacethereof. The trench 105 penetrates through the P-type body region 104,so that a deepest portion thereof reaches the N⁻-type region 103. A gateelectrode 107 made of polysilicon doped with an N-type impurity in ahigh concentration is embedded in the trench 105 through a gateinsulating film 106 made of SiO₂ (silicon oxide).

An N⁺-type source region 108 is formed on a surface layer portion of theP-type body region 104 along the trench 105. A P⁺-type contact region109 is formed at the center of the N⁺-type source region 108 in planview, to penetrate through the N⁺-type source region 108.

An interlayer dielectric film 110 is laminated on the N⁻-type epitaxiallayer 102. A source wire 111 is formed on the interlayer dielectric film110. This source wire 111 is grounded. The source wire 111 is in contact(electrically connected) with the N⁺-type source region 108 and theP⁺-type contact region 109 through a contact hole 112 formed in theinterlayer dielectric film 110. Agate wire 113 is electrically connectedto the gate electrode 107 through another contact hole (not shown)formed in the interlayer dielectric film 110.

A drain electrode 114 is formed on a back surface of the N⁺-typesubstrate 101.

When the potential of the gate electrode 107 is controlled whileapplying a positive voltage of a proper level to the drain electrode114, a channel is formed in the vicinity of an interface between theP-type body region 104 and the gate insulating film 106, and a currentflows between the N⁺-type source region 108 and the drain electrode 114.Thus, a switching operation of the VDMOSFET is achieved.

For example, the product R_(on)·Q_(g) of the on-resistance R_(on) andthe gate charge quantity Q_(g) is employed as an index indicating theswitching performance of the VDMOSFET.

The on-resistance R_(on) is the resistance between a source and a drain.In the semiconductor device 100 shown in FIG. 7, the on-resistanceR_(on) corresponds to the resistance between the N⁺-type source region108 and the N⁺-type substrate 101 (between the source wire 111 and thedrain electrode 114).

The gate charge quantity Q_(g) corresponds to the quantity of chargesstored in a combined capacitance of the gate-to-drain capacitance C_(gd)and a gate-to-source capacitance C_(gs). In the semiconductor device 100shown in FIG. 7, the gate-to-drain capacitance C_(gd) corresponds to acombined capacitance of the capacitance of a portion of the gateinsulating film 106 sandwiched between the gate electrode 107 and thebottom surface of the trench 105 and the capacitance of a depletionlayer 115 spreading from the interface between the N⁻-type region 103and the P-type body region 104. In the semiconductor device 100 shown inFIG. 7, a gate-to-source capacitance C_(gs) corresponds to thecapacitance of a portion of the gate insulating film 106 sandwichedbetween the gate electrode 107 and the N⁺-type source region 108.

The speed of the switching operation can be increased as the productR_(on)·Q_(g) of the on-resistance R_(on) and the gate charge quantityQ_(g) is reduced. As shown in FIG. 8, however, the on-resistance R_(on)and the gate charge quantity Q_(g) are in the so-called trade-offrelation, such that the former is increased when the latter is reducedand vice versa. In order to reduce the product R_(on)·Q_(g), therefore,one of the on-resistance R_(on) and the gate charge quantity Q_(g) mustbe reduced, while increase of the other must be prevented.

FIG. 9 is a schematic sectional view showing the structure of anotherconventional semiconductor device having a trench gate type VDMOSFET.

The semiconductor device 401 includes an N⁺-type substrate 402. AnN⁻-type epitaxial layer 403 is laminated on the N⁺-type substrate 402. Abase layer portion of the N⁻-type epitaxial layer 403 forms an N -typeregion 404, while a P-type body region 405 is formed on a surface layerportion of the N⁻-type epitaxial layer 403 vertically adjacently to theN⁻-type region 404.

A trench 406 is dug in the N⁻-type epitaxial layer 403 from a surfacethereof. The trench 406 penetrates through the P-type body region 405,so that a deepest portion thereof reaches the N⁻-type region 404. A gateelectrode 408 made of polysilicon (doped polysilicon) doped with anN-type impurity in a high concentration is embedded in the trench 406through a gate insulating film 407.

An N⁺-type source region 409 is formed on a surface layer portion of theP-type body region 405 along the trench 406. A P⁺-type source contactregion 410 is formed on the N⁺-type source region 409, to penetratethrough the N⁺-type source region 409.

A drain electrode 415 is formed on a back surface of the N⁺-typesubstrate 402.

When the potential of the gate electrode 408 is controlled whilegrounding the N⁺-type source region 409 and applying a positive voltageof a proper level to the drain electrode 415, a channel is formed in thevicinity of an interface between the P-type body region 405 and the gateinsulating film 407, and a current flows between the N⁺-type sourceregion 409 and the drain electrode 415.

In the steps of manufacturing the semiconductor device 401, a siliconoxide film is formed on the surface of the N⁻-type epitaxial layer 403including the inner surface of the trench 406. Then, the gate electrode408 made of doped polysilicon is formed on the silicon oxide film in thetrench 406. Thereafter HF (hydrofluoric acid) is supplied to the surfaceof the portion of the silicon oxide film located outside the trench 406to remove this portion of the silicon oxide film, in advance of ionimplantation for forming the N⁺-type source region 409.

At this time, the upper portion of the silicon oxide film in the trench406, i.e., the upper end portion of the gate insulating film 407 is alsoremoved with HF, to result in a portion where the gate electrode 408 andthe N⁺-type source region 409 are opposed to each other without throughthe gate insulating film 407, as shown in FIG. 9. In the ionimplantation for forming the N⁺-type source region 409, impurity ionsmay be implanted into not only the N⁻-type epitaxial layer 403 but alsothe gate insulating film 407, to denature the film quality of theportion into which the impurity is implanted. Thus, the conventionalsemiconductor device 401 disadvantageously has a low withstand voltage(gate-to-source withstand voltage) between the gate electrode 408 andthe N⁺-type source region 409.

SUMMARY OF THE INVENTION

A first object of the present invention is to provide a semiconductordevice capable of reducing the gate charge quantity Q_(g) withoutincreasing the on-resistance R_(on) and a method of manufacturing thesame.

A second object of the present invention is to provide a semiconductordevice capable of improving the gate-to-source withstand voltage.

A semiconductor device according to one aspect of the present inventionincludes: a semiconductor layer; a first conductivity type region of afirst conductivity type formed on abase layer portion of thesemiconductor layer; a body region of a second conductivity type formedon the semiconductor layer to be in contact with the first conductivitytype region; a trench formed on the semiconductor layer to penetratethrough the body region so that a deepest portion thereof reaches thefirst conductivity type region; a source region of the firstconductivity type formed on a surface layer portion of the semiconductorlayer around the trench to be in contact with the body region; agateinsulating film formed on a bottom surface and aside surface of thetrench; and agate electrode embedded in the trench through the gateinsulating film. The gate insulating film includes a thick-film portionhaving a relatively large thickness (thickness in the depth direction ofthe trench) on the bottom surface of the trench.

According to this structure, the first conductivity type region isformed on the base layer portion of the semiconductor layer. The bodyregion of the second conductivity type is formed on the semiconductorlayer to be in contact with the first conductivity type region. Further,the trench in which the gate electrode is embedded through the gateinsulating film is formed on the semiconductor layer. The trenchpenetrates through the body region, so that the deepest portion thereofreaches the first conductivity type region. The source region of thefirst conductivity type is formed on the surface layer portion of thesemiconductor layer around the trench. The gate insulating film includesthe thick-film portion having the relatively large thickness on thebottom surface of the trench.

In a trench gate type transistor, a channel is formed on a region(channel forming region) in the vicinity of the interface between a bodyregion and a gate insulating film.

For example, the capacitance between a gate electrode and the bottomsurface of a trench may be reduced by uniformly increasing the thicknessof the gate insulating film thereby increasing an interval between thegate electrode and the bottom surface of the gate electrode. If thethickness of a portion of the gate insulating film opposed to thechannel forming region is increased, however, not only the gatethreshold voltage is increased but also the current capacity is reduced,and hence the channel resistance is increased. Consequently, theon-resistance R_(on) is increased.

When the gate insulating film has the thick-film portion on the bottomsurface of the trench, however, the interval between the gate electrodeand the bottom of the trench opposed to each other through thethick-film portion can be increased while setting the thickness of thegate insulating film on the side surface of the trench to a proper valuein consideration of the gate threshold voltage and the channelresistance. Thus, the capacitance between the gate electrode and thebottom surface (first conductivity type region) of the trench can bereduced without increasing the gate threshold voltage and theon-resistance R_(on). Therefore, the gate-to-drain capacitance C_(gd)can be reduced without increasing the on-resistance R_(on), and the gatecharge quantity Q_(g) can also be reduced. Consequently, the speed ofthe switching operation of the transistor can be increased.

Preferably, the gate insulating film includes the thick-film portion ona peripheral edge portion of the bottom surface of the trench, andincludes a thin-film portion having a relatively small thickness on acentral portion surrounded by the peripheral edge portion. In otherwords, the gate insulating film is preferably relatively thickly formedon the peripheral edge portion of the bottom surface of the trench, andrelatively thinly formed on the central portion surrounded by theperipheral edge portion.

The gate insulating film is so thickly formed on the peripheral edgeportion of the bottom surface of the trench that the interval betweenthe gate electrode and the bottom surface of the trench can be increasedon the peripheral edge portion, and the capacitance therebetween can bereduced. Further, the thickness of the thin-film portion is so set thatthe upper surface of the thin-film portion is positioned to be flushwith or lower than the interface between the first conductivity typeregion and the body region, whereby the gate electrode can be opposed tothe channel forming region along the overall length (total channellength) thereof. Consequently, an excellent operation of the transistorcan be ensured.

Thus, the excellent operation of the transistor can be ensured whilereducing the gate-to-drain capacitance C_(gd).

The semiconductor device having this structure can be obtained by amethod including the steps of: forming a trench on a semiconductor layerof a first conductivity type; forming an oxide film material depositionlayer by depositing an oxide film material on the semiconductor layer;partially leaving the oxide film material deposition layer on aperipheral edge portion of a bottom surface of the trench by etchingback the oxide film material deposition layer; forming an oxide film byoxidizing a surface of the semiconductor layer including the bottomsurface and a side surface of the trench and the oxide film materialdeposition layer left on the peripheral edge portion of the bottomsurface of the trench; forming a gate electrode on the oxide film tofill up the trench; forming a body region of a second conductivity typeby introducing an impurity of the second conductivity type from thesurface of the semiconductor layer; forming a source region of the firstconductivity type to be in contact with the body region by introducingan impurity of the first conductivity type into the periphery of thetrench from the surface of the semiconductor layer; and forming a gateinsulating film on the bottom surface and the side surface of the trenchby removing a portion of the oxide film located outside the trench.

According to this method, the oxide film material deposition layer ispartially left on the peripheral edge portion of the bottom surface ofthe trench. The left oxide film material deposition layer is oxidizedsimultaneously with the surface of the semiconductor layer including thebottom surface and the side surface of the trench. Due to thisoxidation, the left oxide film material deposition layer forms athick-film portion having a relatively large thickness. On the otherhand, a portion formed by oxidizing the bottom surface of the trenchforms a thin-film portion having a relatively small thickness. Thethick-film portion and the thin-film portion form the thick-film portionand the thin-film portion of the gate insulating film respectively.

Thus, the thick-film portion and the thin-film portion of the gateinsulating film can be simultaneously formed. Therefore, the gateinsulating film including the thick-film portion and the thin-filmportion can be easily formed without through complicated steps.

The gate insulating film may have a uniform thickness on the sidesurface of the trench.

In this case, the thickness of the thick-film portion is preferablylarger than the thickness of the gate insulating film on the sidesurface of the trench.

The on-resistance R_(on) can be reduced by relatively reducing thethickness of the gate insulating film on the side surface of the trench,while the gate-to-drain capacitance C_(gd) can be reduced by relativelyincreasing the thickness of the thick-film portion.

The gate insulating film may be so formed that the thickness of aportion in contact with the source region is larger than the thicknessof a portion in contact with the body region on the side surface of thetrench.

The capacitance between the gate electrode and the source region, i.e.,the gate-to-source capacitance C_(gs) can be reduced by increasing thethickness of the portion of the gate insulating film in contact with thesource region while setting the thickness of the portion of the gateinsulating film in contact with the body region to a proper value inconsideration of the gate threshold voltage and the channel resistance.Thus, the gate-to-source capacitance C_(gs) can be reduced withoutincreasing the on-resistance R_(on), and the gate charge quantity Q_(g)can also be reduced. Consequently, the speed of the switching operationof the transistor can be further increased.

The thickness of the portion of the gate insulating film in contact withthe source region is so increased that a large interval can be ensuredbetween the gate electrode and the source region. Consequently, thegate-to-source withstand voltage can be improved.

In this case, the thickness of the thick-film portion is preferablylarger than the thickness of the portion in contact with the bodyregion.

The on-resistance R_(on) can be reduced by relatively reducing thethickness of the portion of the gate insulating film in contact with thebody region, while the gate-to-drain capacitance C_(gd) can be reducedby relatively increasing the thickness of the thick-film portion.

A semiconductor device according to another aspect of the presentinvention includes: a semiconductor layer; a first conductivity typeregion of a first conductivity type formed on a base layer portion ofthe semiconductor layer; a body region of a second conductivity typeformed on the semiconductor layer to be in contact with the firstconductivity type region; a trench formed on the semiconductor layer topenetrate through the body region so that a deepest portion thereofreaches the first conductivity type region; a source region of the firstconductivity type formed on a surface layer portion of the semiconductorlayer around the trench to be in contact with the body region; a gateinsulating film formed on a bottom surface and a side surface of thetrench; and a gate electrode embedded in the trench through the gateinsulating film. The gate insulating film is so formed that thethickness of a portion in contact with the source region is larger thanthe thickness of a portion in contact with the body region on the sidesurface of the trench.

According to this structure, the first conductivity type region isformed on the base layer portion of the semiconductor layer. The bodyregion of the second conductivity type is formed on the semiconductorlayer to be in contact with the first conductivity type region. Further,the trench in which the gate electrode is embedded through the gateinsulating film is formed on the semiconductor layer. The trenchpenetrates through the body region, so that the deepest portion thereofreaches the first conductivity type region. The source region of thefirst conductivity type is formed on the surface layer portion of thesemiconductor layer around the trench. The gate insulating film is soformed that the thickness of the portion in contact with the sourceregion is larger than the thickness of the portion in contact with thebody region on the side surface of the trench. In other words, theportion of the gate insulating film adjacent to the source region has alarger thickness than the remaining portion of the gate insulating film.

The capacitance between the gate electrode and the source region, i.e.,the gate-to-source capacitance C_(gs) can be reduced by increasing thethickness of the portion of the gate insulating film in contact with thesource region while setting the thickness of the portion of the gateinsulating film in contact with the body region to a proper value inconsideration of the gate threshold voltage and the channel resistance.Thus, the gate-to-source capacitance C_(gs) can be reduced withoutincreasing the on-resistance R_(on), and the gate charge quantity Q_(g)can also be reduced. Consequently, the speed of the switching operationof the transistor can be increased.

The thickness of the portion of the gate insulating film in contact withthe source region is so increased that a large interval can be ensuredbetween the gate electrode and the source region. Consequently, thegate-to-source withstand voltage can be improved.

Preferably, the gate insulating film extends from the side surface ofthe trench onto the surface of the source region.

Thus, the gate insulating film can be reliably interposed between thegate electrode and the source region. Consequently, the gate-to-sourcewithstand voltage can be reliably improved.

The foregoing and other objects, features and effects of the presentinvention will become more apparent from the following detaileddescription of the preferred embodiments with reference to the attacheddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of a semiconductor device accordingto a first embodiment of the present invention.

FIG. 2A is a schematic sectional view for describing a method ofmanufacturing the semiconductor device shown in FIG. 1.

FIG. 2B is a schematic sectional view of a step subsequent to that ofFIG. 2A.

FIG. 2C is a schematic sectional view of a step subsequent to that ofFIG. 2B.

FIG. 2D is a schematic sectional view of a step subsequent to that ofFIG. 2C.

FIG. 2E is a schematic sectional view of a step subsequent to that ofFIG. 2D.

FIG. 2F is a schematic sectional view of a step subsequent to that ofFIG. 2E.

FIG. 2G is a schematic sectional view of a step subsequent to that ofFIG. 2F.

FIG. 2H is a schematic sectional view of a step subsequent to that ofFIG. 2G.

FIG. 2I is a schematic sectional view of a step subsequent to that ofFIG. 2H.

FIG. 2J is a schematic sectional view of a step subsequent to that ofFIG. 2I.

FIG. 2K is a schematic sectional view of a step subsequent to that ofFIG. 2J.

FIG. 2L is a schematic sectional view of a step subsequent to that ofFIG. 2K.

FIG. 2M is a schematic sectional view of a step subsequent to that ofFIG. 2L.

FIG. 2N is a schematic sectional view of a step subsequent to that ofFIG. 2M.

FIG. 3 is a schematic sectional view of a semiconductor device accordingto a second embodiment of the present invention.

FIG. 4A is a schematic sectional view successively showing the step ofmanufacturing the semiconductor device shown in FIG. 3.

FIG. 4B is a schematic sectional view of a step subsequent to that ofFIG. 4A.

FIG. 4C is a schematic sectional view of a step subsequent to that ofFIG. 4B.

FIG. 4D is a schematic sectional view of a step subsequent to that ofFIG. 4C.

FIG. 4E is a schematic sectional view of a step subsequent to that ofFIG. 4D.

FIG. 4F is a schematic sectional view of a step subsequent to that ofFIG. 4E.

FIG. 4G is a schematic sectional view of a step subsequent to that ofFIG. 4F.

FIG. 5 is a schematic sectional view of a semiconductor device accordingto a third embodiment of the present invention.

FIG. 6A is a schematic sectional view successively showing the step ofmanufacturing the semiconductor device shown in FIG. 5.

FIG. 6B is a schematic sectional view of a step subsequent to that ofFIG. 6A.

FIG. 6C is a schematic sectional view of a step subsequent to that ofFIG. 6B.

FIG. 6D is a schematic sectional view of a step subsequent to that ofFIG. 6C.

FIG. 6E is a schematic sectional view of a step subsequent to that ofFIG. 6D.

FIG. 6F is a schematic sectional view of a step subsequent to that ofFIG. 6E.

FIG. 6G is a schematic sectional view of a step subsequent to that ofFIG. 6F.

FIG. 6H is a schematic sectional view of a step subsequent to that ofFIG. 6G.

FIG. 6I is a schematic sectional view of a step subsequent to that ofFIG. 6H.

FIG. 6J is a schematic sectional view of a step subsequent to that ofFIG. 6I.

FIG. 6K is a schematic sectional view of a step subsequent to that ofFIG. 6J.

FIG. 6L is a schematic sectional view of a step subsequent to that ofFIG. 6K.

FIG. 7 is a schematic sectional view showing the structure of aconventional semiconductor device having a trench gate type VDMOSFET.

FIG. 8 is a graph showing the relation between the on-resistance R_(on)and the gate charge quantity Q_(g) of the VDMOSFET shown in FIG. 7.

FIG. 9 is a schematic sectional view showing the structure of anotherconventional semiconductor device having a trench gate type VDMOSFET.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is a schematic sectional view of a semiconductor device accordingto a first embodiment of the present invention.

The semiconductor device 1 has an array structure formed by arrangingunit cells having trench gate type VDMOSFET in a form of a matrix.

On an N⁺-type substrate 2 forming a base of the semiconductor device 1,an N⁻-type epitaxial layer 3 made of silicon doped with an N-typeimpurity in a lower concentration (10¹⁵/cm³, for example) than theN⁺-type substrate 2 is laminated. Abase layer portion of the epitaxiallayer 3 forms an N⁻-type region 4 as a first conductivity type region ina state after epitaxy. In the epitaxial layer 3, a P-type body region 5is formed on the N⁻-type region 4 to be in contact with the N⁻-typeregion 4.

A trench 6 is dug in the epitaxial layer 3 from a surface thereof. Thetrench 6 penetrates through the body region 5, so that a deepest portionthereof reaches the N⁻-type region 4. A plurality of such trenches 6 areformed at regular intervals in the horizontal direction in FIG. 1, toextend in a direction (along a gate width) perpendicular to the plane ofFIG. 1 respectively. Each trench 6 has a width W₁ of 0.5 μm, forexample, in the horizontal direction (orthogonal to the gate width) inFIG. 1. A gate insulating film 7 made of SiO₂ (silicon oxide) is formedin the trench 6, to cover an overall inner surface thereof.

The gate insulating film 7 includes a thick-film portion 71 having arelatively large thickness (thickness in a depth direction of the trench6) and a thin-film portion 72 having a relatively small thickness on abottom surface of the trench 6.

The thick-film portion 71 is formed on the peripheral edge portion ofthe bottom surface of the trench 6. The thick-film portion 71 has asectional shape increased in thickness toward a side surface of thetrench 6. The maximum thickness T_(ox2) (150 nm, for example) of thethick-film portion 71 is larger than an interval H₁ (50 nm, for example)between the bottom surface of the trench 6 and an upper surface of theN⁻-type region 4 (interface between the N⁻-type region 4 and the bodyregion 5). On the other hand, the minimum thickness T_(ox1) (50 nm, forexample) of the thick-film portion 71 is equal to the interval H₁. Thethick film 71 is in contact with the bottom surface of the trench 6 witha width W₃ of 0.1 μm, for example.

The thin-film portion 72 is formed at a center of the bottom surface ofthe trench 6. An upper surface of the thin-film portion 72 is flush withthe upper surface of the N⁻-type region 4. In other words, the thicknessT_(ox1) of the thin-film portion 72 is equal to the interval H₁ betweenthe bottom surface of the trench 6 and the upper surface of the N⁻-typeregion 4. The thin-film. portion 72 is in contact with the bottomsurface of the trench 6 with a width W₂ of 0.3 μm, for example.

The gate electrode 8 is embedded in the trench 6 by filling up an innerside of the gate insulating film 7 in the trench 6 with polysilicondoped with an N-type impurity in a high concentration.

On the surface layer portion of the epitaxial layer 3, N⁺-type sourceregions 9 each having a higher N-type impurity concentration (10¹⁹/cm³,for example) than the N⁻-type region 4 are formed on both sides of thetrench 6 in the direction (horizontal direction in FIG. 1) orthogonal tothe gate width. Each source region 9 extends along the trench 6 in thedirection along the gate width, so that a bottom portion thereof is incontact with the body region 5. A P⁺-type contact region 10 is formed ata center of the source region 9 in the direction orthogonal to the gatewidth, to penetrate through the source region 9.

That is, the trenches 6 and the source regions 9 are alternatelyprovided in the direction orthogonal to the gate width, to extend in thedirection along the gate width respectively. Boundaries between the unitcells adjacent to one another in the direction orthogonal to the gatewidth are set on the source regions 9, along the source regions 9. Atleast one or more contact regions 10 are provided over each pair of unitcells adjacent to each other in the direction orthogonal to the gatewidth. Boundaries between the unit cells adjacent to one another in thedirection along the gate width are so set that the gate electrode 8included in each unit cell has a constant gate width.

An interlayer dielectric film 13 is laminated on the epitaxial layer 3.

Source wires 14 and a gate wire 16 are formed on the interlayerdielectric film 13. The source wires 14 are grounded.

In the interlayer dielectric film 13, contact holes 15 are penetratinglyformed on positions where the source wires 14 and the source regions 9including the contact regions 10 are opposed to one another. Sourceplugs 18 are embedded in the contact hole 15. The source wires 14 are incontact (electrically connected) with the source regions 9 and thecontact regions 10 through the source plugs 18.

In the interlayer dielectric film 13, further, a contact hole 11 ispenetratingly formed on the position where the gate wire 16 and the gateelectrode 8 are opposed to each other. A gate plug 12 is embedded in thecontact hole 11. The gate wire 16 is in contact (electrically connected)with the gate electrode 8 through the gate plug 12.

A drain electrode 27 is formed on a back surface of the N⁺-typesubstrate 2.

When the potential of the gate electrode 8 is controlled while applyinga positive voltage of a proper level to the drain electrode 27, channelsare formed in the vicinity (channel forming regions 29) of the interfacebetween the body region 5 and the gate insulating film 7, and currentflows between the source regions 9 and the drain electrode 27.

In the semiconductor device 1, the thickness of the gate insulating film7 is not uniformly increased, but the gate insulating film 7 is locallythickly formed on the peripheral edge portion of the bottom surface ofthe trench 6. Thus, the gate insulating film 7 includes the thick-filmportion 71 having a larger thickness than the remaining portion on theperipheral edge portion of the bottom surface of the trench 6.Therefore, the interval between the gate electrode 8 and the bottomsurface of the trench 6 opposed to each other through the thick-filmportion 71 can be increased while properly setting the thickness of thegate insulating film 7 on the side surface of the trench 6 inconsideration of the gate threshold voltage and the channel resistance.Thus, the capacitance C_(ox1) between the gate electrode 8 and thebottom surface of the trench 6 (N⁻-type region 4) opposed to each otherthrough the gate insulating film 7 can be reduced without increasing thegate threshold voltage and the on-resistance R_(on). Therefore, thegate-to-drain capacitance C_(gd) can be reduced without increasing theon-resistance R_(on), and the gate charge quantity Q_(g) can also bereduced. Consequently, the speed of the switching operation of theVDMOSFET can be increased.

In the semiconductor device 100 shown in FIG. 7, it is assumed that thewidth of the trench 105 and the gate width are equal to the width W₁ ofthe trench 6 and the gate width W_(g) respectively. It is also assumedthat the thickness of the gate insulating film 106 is equal to thethickness T_(ox1) of the thick-film portion 72 of the gate insulatingfilm 7. In this case, the capacitance C_(ox2) between the gate electrode107 and the bottom surface of the trench 105 (N⁻-type region 103)opposed to each other through the gate insulating film 106 is expressedas follows:

C _(ox2)=ε_(ox) ·W ₁ ·W _(g) /T _(ox1)

where ε_(ox) represents the dielectric constant of SiO₂.

Assuming that T_(or(t))) represents the thickness of the thick-filmportion 71 in the depth direction of the trench 6 in the semiconductordevice 1, on the other hand, the capacitance C_(ox1) between the gateelectrode 8 and the bottom surface of the trench 6 (N⁻-type region 4)opposed to each other through the gate insulating film 7 is expressed asfollows:

C _(ox1)=ε_(ox) ·W ₂ ·W _(g) /T _(ox1)+2∫₀ ^(w3)ε_(ox) ·dt/T _(ox(t))

It is assumed that ∫₀ ^(w3)ε_(ox) ·dt/T _(ox(t))≈W₃/2T _(ox1).Substituting W₁=0.5 μm, W₂=0.3 μm, W₃=0.1 μm and T_(ox1)=50 nm in theabove expressions for obtaining the capacitances C_(ox1) and C_(ox2) andcomparing C_(ox1) and C_(ox2) with each other, the relation therebetweenis expressed as follows:

C_(ox1)=0.8C_(ox2)

Hence, it is understood that the capacitance C_(ox1) in the structure ofthe semiconductor device 1 is reduced below the capacitance C_(ox2) inthe structure of the semiconductor device 100.

The gate insulating film 7 includes the thin-film portion 72 at thecenter of the bottom surface of the trench 6, and the upper surface ofthe thin-film portion 72 is flush with the upper surface of the N⁻-typeregion 4. Therefore, the gate electrode 8 can be opposed to the overallwidths (total channel lengths) of the channel forming regions 29.Consequently, an excellent operation of the VDMOSFET can be ensured.

Thus, the excellent operation of the VDMOSFET can be ensured whilereducing the gate-to-drain capacitance C_(gd).

FIGS. 2A to 2N are schematic sectional views for describing a method ofmanufacturing the semiconductor device shown in FIG. 1.

First, the epitaxial layer 3 is formed on the N⁺-type substrate 2 byepitaxy, as shown in FIG. 2A.

Then, a sacrificial oxide film 21 made of SiO₂ is formed on the surfaceof the epitaxial layer 3 by thermal oxidation, as shown in FIG. 2B.Thereafter an SiN (silicon nitride) layer is formed on the sacrificialoxide film 21 by P-CVD (Plasma Chemical Vapor Deposition) and patterned,to form a hard mask 22 having an opening in a portion opposed to theportion for forming the trench 6. The sacrificial layer 21 and theepitaxial layer 3 are etched through the hard mask 22, thereby formingthe trench 6 (step of forming a trench). After the formation of thetrench 6, the sacrificial oxide film 21 and the hard mask 22 areremoved.

Then, an oxide film 23 made of SiO₂ is formed on the overall region ofthe surface of the epitaxial layer 3 including the inner surface of thetrench 6 by thermal oxidation, as shown in FIG. 2C.

After the formation of the oxide film 23, a deposition layer 24 ofpolysilicon doped with an N-type impurity in a high concentration isformed on the oxide film 23 by CVD, as shown in FIG. 2D (step of formingan oxide film material deposition layer). The deposition layer 24 isformed in a thickness not completely filling up the trench 6.

Then, the deposition layer 24 is etched back, as shown in FIG. 2E. Dueto this etch-back, the deposition layer 24 partially remains on theperipheral edge portion of the bottom surface of the trench 6 as adeposit section 30 (step of partially leaving the oxide film materialdeposition layer).

Then, the oxide film 23 is removed by etching, to leave portionssandwiched between the deposit section 30 and the inner surface of thetrench 6, as shown in FIG. 2F. Thus, the central portion of the bottomsurface and the side surface of the trench 6 are exposed.

Then, the surface of the epitaxial layer 3 and the deposit section 30are oxidized by thermal oxidation to form an oxide film 31, as shown inFIG. 2G. The deposit section 30 made of polysilicon doped with theN-type impurity in a high concentration is oxidized at a rate ofoxidation of three times, for example, as compared with the epitaxiallayer 3 made of Si. Therefore, the surface of the epitaxial layer 3 andthe deposit section 30 are so simultaneously oxidized that a thick-filmportion 32 having a relatively large thickness in the depth direction ofthe trench 6 is formed on the peripheral edge portion of the bottomsurface of the trench 6 while a thin-film portion 33 having a relativelysmall thickness is formed on the central portion surrounded by thisperipheral edge portion.

Then, a deposition layer of polysilicon doped with an N-type impurity ina high concentration is formed on the oxide film 31 by CVD. The trench 6is filled up with the deposition layer of polysilicon. Then, a portionof the deposition layer of polysilicon located outside the trench 6 isremoved by etching. Thus, the gate electrode 8 embedded in the trench 6is obtained, as shown in FIG. 2H (step of forming a gate electrode).

Thereafter ions of a p-type impurity are implanted into the epitaxiallayer 3 from the surface of the oxide film 31. Then, drive-in diffusionis performed. The ions of the P-type impurity implanted into theepitaxial layer 3 are diffused due to this drive-in diffusion, to formthe body region 5 in the epitaxial layer 3, as shown in FIG. 2I (step offorming a body region). The remaining portion of the epitaxial layer 3other than the body region 5 forms the N⁻-type region 4 in the stateafter the epitaxy.

After the drive-in diffusion, a mask 25 having openings in portionsopposed to the portions for forming the source regions 9 is formed onthe oxide film 31, as shown in FIG. 2J. Then, ions of an N-type impurityare implanted into the surface layer portion of the epitaxial layer 3through the openings of the mask 25. After this ion implantation, themask 25 is removed.

Further, another mask 26 having openings in portions opposed to theportions for forming the contact regions 10 is formed on the oxide film31, as shown in FIG. 2K. Then, ions of a P-type impurity are implantedinto the surface layer portion of the epitaxial layer 3 through theopenings of the mask 26. After this ion implantation, the mask 26 isremoved.

Thereafter annealing is performed. The ions of the N-type and P-typeimpurities implanted into the surface layer portion of the epitaxiallayer 3 are activated due to this annealing, to form the source regions9 and the contact regions 10 on the surface layer portion of theepitaxial layer 3, as shown in FIG. 2L (step of forming a sourceregion).

After the aforementioned steps, a portion of the oxide film 31 locatedoutside the trench 6 is removed to leave the oxide film 31 only on theinner surface of the trench 6, thereby obtaining the gate insulatingfilm 7. The thick-film portion 32 and the thin-film portion 33 of theoxide film 31 form the thick-film portion 71 and the thin-film portion72 of the gate insulating film 7 respectively.

Thereafter SiO₂ is deposited on the epitaxial layer 3 by CVD. Then, amask 20 having openings in portions opposed to the portions for formingthe contact holes 11 and 15 respectively is formed on the depositedSiO₂, and SiO₂ is dry-etched through this mask 20. Thus, the interlayerdielectric film 13 having the contact holes 11 and 15 is formed, asshown in FIG. 2M.

Then, the gate plug 12, the gate wire 16, the source plugs 18, thesource wires 14 and the drain electrode 27 are formed, as shown in FIG.2N. Thus, the semiconductor device 1 shown in FIG. 1 is obtained.

According to this method, the deposition layer 24 is so etched back asto partially remain on the peripheral edge portion of the bottom surfaceof the trench 6 as the deposit section 30. The deposit section 30 isoxidized simultaneously with the surface of the epitaxial layer 3 bythermal oxidation. Thus, the deposit section 30 forms the thick-filmportion 32 having the relatively large thickness. On the other hand, theportion resulting from the oxidation of the bottom surface of the trench6 forms the thin-film portion 33 having the relatively small thickness.The thick-film portion 32 and the thin-film portion 33 form thethick-film portion 71 and the thin-film portion 72 of the gateinsulating film 7 respectively.

Thus, the thick-film portion 71 and the thin-film portion 72 of the gateinsulating film 7 can be formed at the same time. That is, the gateinsulating film 7 including the thick-film portion 71 and the thin-filmportion 72 can be easily formed without through complicated steps.

FIG. 3 is a schematic sectional view of a semiconductor device accordingto a second embodiment of the present invention.

Referring to FIG. 3, portions corresponding to those shown in FIG. 1 aredenoted by the same reference numerals. In the following, only pointsdifferent from those of the structure shown in FIG. 1 are described, andredundant description is omitted as to the portions having the samereference numerals.

In the semiconductor device 201 shown in FIG. 3, a gate insulating film7 is so formed that the thickness of a portion 202 in contact with eachsource region 9 is larger than the thickness of a portion 203 in contactwith a body region 5 on the side surface of a trench 6. Morespecifically, the thickness of the portion 202 of the gate insulatingfilm 7 in contact with the source region 9 is increased while thethickness of the portion 203 of the gate insulating film 7 in contactwith the body region 5 is properly set in consideration of the gatethreshold voltage and the channel resistance. As illustrated in FIG. 3,the width of the gate electrode 8 at at least a portion thereof opposingthe source region 9 is smaller than the width of the gate electrode 8 ata portion not opposing the source region 9, the widths being measured ina direction in which the portion of the gate electrode 8 opposes thesource region 9. Thus, the capacitance between a gate electrode 8 andthe source region 9, i.e., the gate-to-source capacitance Cgs can bereduced without increasing the on-resistance Ron, and the gate chargequantity Qg can also be reduced. Consequently, the speed of a switchingoperation of a transistor can be further increased.

Further, the thickness of the portion 202 of the gate insulating film 7in contact with the source region 9 is so increased that a largeinterval can be ensured between the gate electrode 8 and the sourceregion 9. Consequently, the gate-to-source withstand voltage can beimproved.

FIGS. 4A to 4G are schematic sectional views for describing a method ofmanufacturing the semiconductor device shown in FIG. 3.

The semiconductor device 201 shown in FIG. 3 is obtained by carrying outthe steps shown in FIGS. 2A to 2G and thereafter carrying out the stepsshown in FIGS. 4A to 4G in place of the steps shown in FIGS. 2H to 2M.Referring to FIGS. 4A to 4G, portions corresponding to those shown inFIGS. 21 to 2M are denoted by the same reference numerals.

After the step shown in FIG. 2G, a deposition layer of polysilicon dopedwith an N-type impurity in a high concentration is formed on an oxidefilm 31 by CVD. The trench 6 is filled up with the deposition layer ofpolysilicon. Then, portions of the deposition layer of polysilicon andthe oxide film 31 located outside the trench 6 are removed by etching.Thus, the gate electrode 8 embedded in the trench 6 is obtained, asshown in FIG. 4A.

Thereafter ions of a P-type impurity are implanted into an epitaxiallayer 3 from the surface of the oxide film 31. Then, drive-in diffusionis performed. The ions of the P-type impurity implanted into theepitaxial layer 3 are diffused due to this drive-in diffusion, therebyforming the body region 5 in the epitaxial layer 3. Further, theremaining portion of the epitaxial layer 3 other than the body region 5forms an N⁻-type region 4 in a state after epitaxy.

After the drive-in diffusion, a mask 25 having an opening in a portionopposed to a portion for forming the source region 9 is formed on theoxide film 31, as shown in FIG. 4B. Then, ions of an N-type impurity areimplanted into the surface layer portion of the epitaxial layer 3through the opening of the mask 25. After this ion implantation, themask 25 is removed.

Further, another mask 26 having an opening in a portion opposed to aportion for forming each contact region 10 is formed on the oxide film31, as shown in FIG. 4C. Then, ions of a P-type impurity are implantedinto the surface layer portion of the epitaxial layer 3 through theopening of the mask 26. After this ion implantation, the mask 26 isremoved.

Thereafter annealing is performed. The ions of the N-type and P-typeimpurities implanted into the surface layer portion of the epitaxiallayer 3 are activated due to this annealing, to form the source region 9and the contact region 10 on the surface layer portion of the epitaxiallayer 3, as shown in FIG. 4D.

Then, HF (hydrofluoric acid) is supplied to the surface of the epitaxiallayer 3. A portion of the oxide film 31 in contact with the sourceregion 9 is removed due to the action of HF, as shown in FIG. 4E.

Thereafter an oxide film 204 is formed on the surfaces of the epitaxiallayer 3 and the gate electrode 8 and between the source region 9 and thegate electrode 8 by thermal oxidation, as shown in FIG. 4F. The portioninto which the N-type impurity is implanted in a high concentration hasa high rate for growing an oxide film by thermal oxidation, and hencethe oxide film 204 is grown into a larger thickness than the oxide film31 in a short time.

After the aforementioned steps, a portion of the oxide film 204 locatedoutside the trench 6 is removed so that the oxide film 204 remains onlyin the trench 6, as shown in FIG. 4G. The remaining oxide film 204constitutes the gate insulating film 7 along with the oxide film 31, andforms the portion 202 of the gate insulating film 7 in contact with thesource region 9. Then, SiO₂ is deposited on the epitaxial layer 3 byCVD. Then, a mask 20 having openings in portions opposed to portions forforming contact holes 11 and 15 respectively is formed on the depositedSiO₂, and SiO₂ is dry-etched through this mask 20. Thus, an interlayerdielectric film 13 having the contact holes 11 and 15 is formed.

Then, a gate plug 12, a gate wire 16, source plugs 18, source wires 14and a drain electrode 27 are formed. Thus, the semiconductor device 201shown in FIG. 3 is obtained.

FIG. 5 is a schematic sectional view of a semiconductor device accordingto a third embodiment of the present invention.

The semiconductor device 301 has an array structure formed by arrangingunit cells having trench gate type VDMOSFETS in the form of a matrix.

On an N⁺-type substrate 302 forming the base of the semiconductor device301, an N⁻-type epitaxial layer 303 made of silicon doped with an N-typeimpurity in a lower concentration (10¹⁵cm³, for example) than theN⁺-type substrate 302 is laminated as a semiconductor layer. A baselayer portion of the epitaxial layer 303 forms an N -type region 304 asa first conductivity type region in a state after epitaxy. In theepitaxial layer 303, a P-type body region 305 is formed on the N⁻-typeregion 304, to be in contact with the N⁻-type region 304.

A trench 306 is dug in the epitaxial layer 303 from the surface thereof.The trench 306 penetrates through the body region 305, so that thedeepest portion thereof reaches the N⁻-type region 304. A plurality ofsuch trenches 306 are formed at regular intervals in a horizontaldirection in FIG. 5, to extend in a direction (along the gate width)orthogonal to the plane of FIG. 5 respectively.

Agate electrode 308 made of polysilicon doped with an N-type impurity ina high concentration is embedded in the trench 306 through a gateinsulating film 307. The gate insulating film 307 is so formed as tocover a bottom surface and a side surface of the trench 306, to extendfrom the side surface of the trench 306 onto a surface of the epitaxiallayer 303, and to cover a surface of the gate electrode 308.

On the surface layer portion of the epitaxial layer 303, N⁺-type sourceregions 309 each having a higher N-type impurity concentration(10¹⁹/cm³, for example) than the N⁻-type region 304 are formed on bothsides of the trench 306 in the direction (horizontal direction in FIG.5) orthogonal to the gate width. Each source region 309 extends alongthe trench 306 in the direction along the gate width, so that a bottomportion thereof is in contact with the body region 305. AP⁺-type contactregion 310 is formed at a center of the source region 309 in thedirection orthogonal to the gate width, to penetrate through the sourceregion 309.

That is, the trenches 306 and the source regions 309 are alternatelyprovided in the direction orthogonal to the gate width, to extend in thedirection along the gate width respectively. Boundaries between the unitcells adjacent to one another in the direction orthogonal to the gatewidth are set on the source regions 309, along the source regions 309.At least one or more contact regions 310 are provided over each pair ofunit cells adjacent to each other in the direction orthogonal to thegate width. Boundaries between the unit cells adjacent to one another inthe direction along the gate width are so set that the gate electrode308 included in each unit cell has the constant gate width.

An interlayer dielectric film 313 is laminated on the epitaxial layer303. Source wires 314 are formed on the interlayer dielectric film 313.The source wires 314 are grounded. The source wires 314 are in contact(electrically connected) with the source regions 309 and the contactregions 310 through contact holes 315 formed in the interlayerdielectric film 313.

A drain electrode 317 is formed on a back surface of the N⁺-typesubstrate 302.

When a potential of the gate electrode 308 is controlled while applyinga positive voltage of a proper level to the drain electrode 317,channels are formed in the vicinity of an interface between the bodyregion 305 and the gate insulating film 307, and current flows betweenthe source regions 309 and the drain electrode 317.

The gate insulating film 307 is so formed that a thickness of a portion318 in contact with each source region 309 is larger than a thickness ofa portion 319 in contact with the body region 305 on the side surface ofthe trench 306. More specifically, the thickness of the portion of thegate insulating film 307 in contact with the source region 309 isincreased while the thickness of the portion 319 of the gate insulatingfilm 307 in contact with the body region 305 is properly setinconsideration of the gate threshold voltage and the channelresistance.

Thus, the capacitance between the gate electrode 308 and the sourceregion 309, i.e., the gate-to-source capacitance C_(gs) can be reducedwithout increasing the on-resistance R_(on), and the gate chargequantity Q_(g) can also be reduced. Consequently, the speed of aswitching operation of a transistor can be further increased.

Further, the thickness of the portion 318 of the gate insulating film307 in contact with the source region 309 is so increased that a largeinterval can be ensured between the gate electrode 308 and the sourceregion 309. Consequently, the gate-to-source withstand voltage can beimproved.

In addition, the gate insulating film 307 extends from the side surfaceof the trench 306 onto a surface of the source region 309. Thus, thegate insulating film 307 can be reliably interposed between the gateelectrode 308 and the source region 309. Consequently, thegate-to-source withstand voltage can be reliably improved.

FIGS. 6A to 6L are schematic sectional views for describing a method ofmanufacturing the semiconductor device shown in FIG. 5.

First, the epitaxial layer 303 is formed on the N⁺-type substrate 302 byepitaxy, as shown in FIG. 6A. Then, a sacrificial oxide film 321 made ofSiO₂ (silicon oxide) is formed on the surface of the epitaxial layer 303by thermal oxidation. Thereafter an SiN (silicon nitride) layer isformed on the sacrificial oxide film 321 by P-CVD (Plasma Chemical VaporDeposition) or LP-CVD (Low Pressure Chemical Vapor Deposition) andpatterned, to form a hard mask 322 having an opening in a portionopposed to a portion for forming the trench 306. The sacrificial layer321 and the epitaxial layer 303 are etched through the hard mask 322,thereby forming the trench 306.

Then, thermal oxidation is performed while leaving the hard mask 322 onthe sacrificial oxide film 321, thereby forming another sacrificialoxide film 323 made of SiO₂ on the inner surface of the trench 306, asshown in FIG. 6B.

Then, the hard mask 322 is removed, as shown in FIG. 6C. Further, thesacrificial oxide films 321 and 323 are removed. Thus, the surface ofthe epitaxial layer 303 is exposed.

Thereafter an oxide film 324 made of SiO₂ is formed on the overallregion of the surface of the epitaxial layer 303 including the innersurface of the trench 306 by thermal oxidation, as shown in FIG. 6D.

Then, a deposition layer of polysilicon doped with an N-type impurity ina high concentration is formed on the oxide film 324 by CVD. The trench306 is filled up with the deposition layer of polysilicon. Then,portions of the oxide film 324 and the deposition layer of polysiliconlocated outside the trench 306 are removed by etching. Thus, the gateelectrode 308 embedded in the trench 306 is obtained, as shown in FIG.6E.

Thereafter ions of a P-type impurity are implanted into the epitaxiallayer 303 from the surface of the epitaxial layer 303, as shown in FIG.6F.

Then, drive-in diffusion is performed. The body region 305 is formed inthe epitaxial layer 303 due to this drive-in diffusion, as shown in FIG.6G.

After the drive-in diffusion, a mask 325 having a pattern coveringportions for forming the contact regions 310 is formed on the epitaxiallayer 303, as shown in FIG. 6H. Then, ions of an N-type impurity areimplanted into the surface layer portion of the epitaxial layer 303through openings of the mask 325. After this ion implantation, the mask325 is removed.

Further, another mask 326 having openings in portions opposed to theportions for forming contact regions 310 is formed on the oxide film324, as shown in FIG. 6I. Then, ions of a P-type impurity are implantedinto the surface layer portion of the epitaxial layer 303 through theopenings of the mask 326. After this ion implantation, the mask 326 isremoved.

Thereafter annealing is performed. The ions of the N-type and P-typeimpurities implanted into the surface layer portion of the epitaxiallayer 303 are activated due to this annealing, to form the sourceregions 309 and the contact regions 310 on the surface layer portion ofthe epitaxial layer 303, as shown in FIG. 6J.

Then, HF (hydrofluoric acid) is supplied to the surface of the epitaxiallayer 303. Portions of the oxide film 324 in contact with the sourceregions 309 are removed due to the action of HF, as shown in FIG. 6K.

Thereafter an oxide film 312 is formed on the surfaces of the epitaxiallayer 303 and the gate electrode 308 and between the source regions 309and the gate electrode 308 by thermal oxidation, as shown in FIG. 6L.The portion into which the N-type impurity is implanted in a highconcentration has a high rate for growing an oxide film by thermaloxidation, and hence the oxide film 312 is grown into a larger thicknessthan the oxide film 324 in a short time. This oxide film 312 isintegrated with the oxide film 324, to constitute the gate insulatingfilm 307 along with the oxide film 324.

After the aforementioned steps, the interlayer dielectric film 313 islaminated on the epitaxial layer 303 by CVD. Then, the contact holes 315etc. are formed in the interlayer dielectric film 313 by etching.Thereafter the source wires 314, a gate wire 16 and the drain electrode317 are formed, whereby the semiconductor device 301 shown in FIG. 5 isobtained.

The gate insulating film 307 (oxide film 312) is formed after theformation of the source regions 309, whereby no N-type impurity isimplanted into the gate insulating film 307. Therefore, the gateinsulating film 307 is not denatured by impurity implantation, and thedielectric voltage is not reduced due to denaturing of the gateinsulating film 307.

The conductivity types of the semiconductor portions of thesemiconductor devices 1, 201 and 301 may be reversed. In other words,the P-type portions maybe converted to N-type portions and vice versa inthe semiconductor devices 1, 201 and 301.

While the present invention has been described in detail by way of theembodiments thereof, it should be understood that these embodiments aremerely illustrative of the technical principles of the present inventionbut not limitative of the invention. The spirit ad scope of the presentinvention are to be limited only by the appended claims.

What is claimed is:
 1. A semiconductor device comprising: asemiconductor substrate of a first conductivity type; a body region of asecond conductivity type formed above the semiconductor substrate; asource region of the first conductivity type being formed on a surfacelayer portion of the body region to be in contact with the body region;a trench penetrating through the source region and the body region; agate insulating film formed on a bottom surface and a side surface ofthe trench; and a gate electrode embedded in the trench through the gateinsulating film, wherein the gate insulating film has a first thicknessat a first portion thereof in contact with the source region and asecond thickness less than the first thickness at a second portionthereof in contact with the body region, the first portion of the gateinsulating film has a first film face in contact with the source region,the second portion of the gate insulating film has a second film face incontact with the body region, the second film face is located at aposition shifted from the first film face in a direction parallel to thebottom surface of the trench, and the first film face and the secondfilm face are in parallel with each other, and are both inclined at thesame slope with respect to the bottom surface of the trench.
 2. Thesemiconductor device according to claim 1, wherein the first film faceextends substantially linearly when viewed in a cross section of thedevice, and the second film face extends substantially linearly whenviewed in the cross section of the device.
 3. The semiconductor deviceaccording to claim 1, wherein the second film face extends longer thanthe first film face when viewed in a cross section of the device.
 4. Thesemiconductor device according to claim 1, wherein the first portion ofthe gate insulating film has a third film face in contact with the gateelectrode at a first gate electrode portion thereof opposing the sourceregion through the first portion of the gate insulating film, the secondportion of the gate insulating film has a fourth film face in contactwith the gate electrode at a second gate electrode portion thereofopposing the body region through the second portion of the gateinsulating film, the fourth film face is located at a position shiftedfrom the third film face in the direction parallel to the bottom surfaceof the trench, and the third film face and the fourth film face are inparallel with each other, and are both inclined at the same slope withrespect to the bottom surface of the trench.
 5. The semiconductor deviceaccording to claim 3, wherein the third film face extends substantiallylinearly when viewed in a cross section of the device, and the forthfilm face extends substantially linearly when viewed in the crosssection of the device.
 6. The semiconductor device according to claim 3,wherein the fourth film face extends longer than the third film facewhen viewed in a cross section of the device.